The invention relates to the detection of base contaminants in air, especially amine contaminants, and to systems employing such detection, including semiconductor fabrication systems and systems for filtering air for semiconductor fabrication and other processes that require uncontaminated air of high quality.
A particular purpose of the invention is to reliably measure low concentrations of airborne base contaminants in a semiconductor manufacturing environment that may adversely affect base-sensitive photolithographic processes being employed.
In semiconductor manufacturing it has been found desirable to detect an organic amine such as NMP (normal methyl pyrrolidinone) or ammonia. Such a contaminant may interfere for instance with a photolithography process used in semiconductor fabrication. The base contaminant may react with protons produced as a result of exposure of a photoresist layer to light. This can interfere with proper exposure and can harm the yield of the process and the rate of production of the semiconductor wafers.
For this reason, semiconductor manufacturers have sought to measure and control the concentration of airborne molecular contamination during the critical steps of the photolithography process which are sensitive to it. A detecting instrument specific to the detection of NMP and a detecting instrument specific to the detection of ammonia have been employed in semiconductor manufacturing facilities to monitor the air quality in the vicinity of production tools.
To understand the novel aspects of the invention it is useful to mention some detection techniques that have been used in other contexts.
For study of combustion processes or atmospheric pollution, U.S. Pat. Nos. 4,333,735 and 3,647,387 disclose processes for measuring the total fixed gaseous nitrogen species, including NH.sub.3, NO, NO.sub.2, HCN and organic amines in gaseous mixtures. The process involves catalytic conversion at elevated temperature of all fixed nitrogen species to NO, followed by chemiluminescent measurement of the resulting NO concentration.
For detection of ammonia, NO and NO.sub.x, U.S. Pat. No. 3,904,371 and products from various instrument manufacturers, including Thermo-Environmental, Advance Pollution Instruments, and Instrumatics International, employ an ammonia scrubber or absorber coupled with a thermal/catalytic converter with or without a molybdenum catalyst. For instance, in the Instrumatics instrument for stack gas analysis, a diluted sample is directed by a valve to alternatively flow through or past an absorber that specifically removes ammonia. The alternating samples proceed along a common line through a thermal converter to a chemiluminescent detector that operates in the 650-750 millibar range. By subtracting signals, the ammonia concentration can be calculated.
U.S. Pat. No. 5,057,436 discloses the use of an ammonia scrubber, positioned between two ammonia detectors, to measure the ammonia level in air.
Another aspect of the invention relates to the use of air filters for the ambient air in semiconductor manufacturing. To avoid harm to the process from NMP or ammonia, semiconductor manufacturers have used chemical filters to remove the contaminants. These filtering systems employ filter stages within an enclosure, the filter media of each stage being penetrable by air with acceptable pressure drop. As air flows through the filtering system, unwanted contaminants are retained on the chemically active surface of the various stages of the filter system. A problem associated with such filtering systems has been to accurately predict the remaining life of the filter so that the filter media can be changed at appropriate times with minimal disruption to the use of the expensive production facility. In the case of semiconductor fabrication facilities, typically, filter life has been estimated by measuring the concentration of ammonia in the air flow associated with the filter system.